This invention relates to surgical implants and, more particularly, to a method of manufacturing such implants from surgical grade austenitic stainless steel such as Type 316L stainless steel. The invention further relates to bone plates and similar implant devices made from Type 316L stainless steel.
Among the biocompatible alloys commonly used for surgical implants are (1) titanium alloys, (2) cobaltchromium-molybdenum-carbon alloys and (3) austenitic stainless steel alloys, the later also known as the 300 series stainless steel.
Of the 300 series stainless steels, Types 316 and 316L are the most commonly used for surgical implants. Type 316L is preferred over Type 316 for implants because of its superior corrosion resistance. Of these three major alloy groups, the titanium alloys, typically Ti-6A1-4V, are the most expensive, and they also exhibit the greatest resistance to corrosion when subjected to the highly corrosive environment of the human body under repeated loading resulting from body motion and weight.
Titanium alloys are useful for implants because they have superior corrosion resistance over other biocompatible materials. However, they have inferior wear characteristics when compared with either the cobalt-chromium-molybdenum alloys or the 300 series stainless steels. For this reason, they have not normally been used for implant which include members having a frictional interface. See Devine, T.M., The Comparative Crevice Corrosion Resistance of Co-Cr Base Surgical Implant Alloys, 123 J. Electochem Soc.: Electrochemical Sc. and Tech. No. 10, pp. 1433-1437 (Oct. 1976). Titanium alloys are also among the more expensive of the implant metals, considering both the raw material and manufacturing costs.
The wrought cobalt-chromium-molybdenum alloys have about the same tensile strength as the titanium alloys, but are generally less corrosion resistant. They also have the further disadvantage of being difficult to work thus requiring long production lead times.
The austenitic or 300 series stainless steels were developed to provide high-strength properties while maintaining workability. These steels are, however, less resistant to corrosion and hence more susceptible to corrosion fatigue than the more expensive titanium alloys and the cobalt-chromium-molybdenum-carbon alloys. Thus, although sufficiently strong for some corrosive environments, their susceptibility to corrosion fatigue has in the past limited their utility for surgical implants. More particularly, their use has in the past been limited to applications which permit the implant device to be designed for mechanical strength or where strength requirement were low to moderate. It would therefore be desirable for surgical implant applications to improve the corrosion resistance of the more workable, more wear resistant, and less expensive, austenitic stainless steels.
In the past, a small amount of cold-working has been done to austenitic stainless steels to increase their mechanical strength. The cold-worked material is then used as a starting material for the manufacture of surgical implants. Additional strength improvement has been reported for one of the austenitic steels, namely Type 316L, by subjecting the cold-worked steel to a low temperature stress relief process, as discussed in "Improved Properties of Type 316L Stainless Steel Implants by Low-Temperature Stress Relief", by Hochman, et al., Journal of Materials at 425-442 (1966). The Hochman, et al. article reports improvement in hardness, tensile strength, and yield strength by stress relieving cold-worked specimens of Type 316L stainless steel at temperatures of about 750.degree. F. (399.degree. C.) for approximately two hours. Although some improvement in mechanical strength of the cold-worked starting material has been achieved by this stress-relief technique, as reported by Hochman, the corrosion fatigue resistance of the stress-relieved starting material is not improved by such stress relieving.
It has also been reported that cold-working austenitic stainless steels reduces their corrosion resistance and therefore makes them more susceptible to pitting and corrosion fatigue in the generally saline environment of the human body. See, e.g. A. Cigada, et al., "Influence of Cold Plastic Deformation on Critical Pitting Potential of AISI 316L Steels in an Artificial Physiological Solution Simulating the Aggressiveness of the Human Body", J. Biomed. Mater. Res. 503 (1977); R.S. Brown, "The Three-Way Tradeoff in Stainless-Steel Selection", Journal of Mechanical Engineering, Nov., 1982, p. 59; and B. Syrett, et al., "Pitting Resistance of New and Conventional Orthopedic Implant Materials - Effect of Metallurgical Corrosion", Vol. 34, No. 4, pp. 138-145 at p. 144 (April 1978). These conclusions appear to be based on corrosion tests of samples of the starting material which had been nominally cold-worked for the purpose of improving its tensile strength over that of the annealed starting material.
Because of the general belief that cold-working would be detrimental to corrosion resistance, implants from austenitic stainless steels such as Type 316L have previously been formed by the process of hot forging. It is generally known that hot forging or heat treating austenitic stainless steels in contrast with the titanium alloys does not improve their tensile strength, although it may slightly improve their corrosion characteristics. Thus the finished implant produced by hot forging did not normally exhibit properties substantially different from the starting material. In addition, fabrication by hot forging is more time consuming and expensive than cold-forging fabrication techniques.
It would therefore be advantageous to provide an inexpensive manufacturing technique for forming implants from the less expensive and better wearing austenitic stainless steels such as Type 316L. It would also be advantageous to improve their resistance to corrosion fatigue to permit their use in a wider range of implant devices than has heretofore been possible.